mash: Make system tray bubble close when another window is activated

mash/wm/public/interfaces/container.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module mash.wm.mojom;
 
+// Containers for windows. The containers are built in a tree; they are not just
+// stacked in this order. See RootWindowController::CreateContainers() for the
+// hierarchy. 
 enum Container {
   ROOT = 0,
   ALL_USER_BACKGROUND,
   USER_WORKSPACE,
   USER_BACKGROUND,
   USER_PRIVATE,
   USER_WINDOWS,
   USER_STICKY_WINDOWS,
   USER_PRESENTATION_WINDOWS,
   USER_SHELF,
   LOGIN_WINDOWS,
   LOGIN_APP,    // TODO(beng): what about dialog boxes login opens?
   LOGIN_SHELF,
+  // Bubble windows, for example the sysui system tray bubble and the screen
+  // brightness bubble.
+  BUBBLES,
   SYSTEM_MODAL_WINDOWS,
   KEYBOARD,
   MENUS,
   TOOLTIPS,
   COUNT
 };
 
 const string kWindowContainer_Property = "ash:window-container";